JILA Physicists Discover 'Quantum Droplet' in Semiconductor

Artist's conception of microscopic "quantum droplet" 

discovered by JILA physicists in a gallium-arsenide 

semiconductor excited by an ultrafast red laser pulse. 

Each droplet consists of electrons and holes 

(representing absent electrons) arranged in a 

liquid-like pattern of rings. The surrounding area 

is plasma. The discovery adds to understanding of 

how electrons interact in optoelectronic devices.

Credit: Baxley/JILA

JILA physicists used an ultrafast laser and help from German theorists to discover a new semiconductor quasiparticle—a handful of smaller particles that briefly condense into a liquid-like droplet.

Quasiparticles are composites of smaller particles that can be created inside solid materials and act together in a predictable way. A simple example is the exciton, a pairing, due to electrostatic forces, of an electron and a so-called "hole," a place in the material's energy structure where an electron could be, but isn't.

The new quasiparticle, described in the Feb. 27, 2014, issue of Nature* and featured on the journal's cover, is a microscopic complex of electrons and holes in a new, unpaired arrangement. The researchers call this a "quantum droplet" because it has quantum characteristics such as well-ordered energy levels, but also has some of the characteristics of a liquid. It can have ripples, for example. It differs from a familiar liquid like water because the quantum droplet has a finite size, beyond which the association between electrons and holes disappears.

Although its lifetime is only a fleeting 25 picoseconds (trillionths of a second), the quantum droplet is stable enough for research on how light interacts with specialized forms of matter.

"Electron-hole droplets are known in semiconductors, but they usually contain thousands to millions of electrons and holes," says JILA physicist Steven Cundiff, who studies the properties of cutting-edge lasers and what they reveal about matter. "Here we are talking about droplets with around five electrons and five holes.

"Regarding practical benefits, nobody is going to build a quantum droplet widget. But this does have indirect benefits in terms of improving our understanding of how electrons interact in various situations, including in optoelectronic devices."

The JILA team created the new quasiparticle by exciting a gallium-arsenide semiconductor with an ultrafast red laser emitting about 100 million pulses per second. The pulses initially form excitons, which are known to travel around in semiconductors. As laser pulse intensity increases, more electron-hole pairs are created, with quantum droplets developing when the exciton density reaches a certain level. At that point, the pairing disappears and a few electrons take up positions relative to a given hole. The negatively charged electrons and positively charged holes create a neutral droplet. The droplets are like bubbles held together briefly by pressure from the surrounding plasma.

JILA's experimental data on energy levels of individual droplet rings agreed with theoretical calculations by co-authors at the University of Marburg in Germany. JILA researchers found they could tap into each energy level by tailoring the quantum properties of the laser pulses to match the particle correlations within the droplets. The droplets seem stable enough for future systematic studies on interactions between light and highly correlated states of matter. In addition, quasiparticles, in general, can have exotic properties not found in their constituent parts, and thus, can play a role in controlling the behavior of larger systems and devices.

JILA is a joint institute of the National Institute of Standards and Technology (NIST) and University of Colorado Boulder. Cundiff is a NIST physicist. The JILA research is supported by the National Science Foundation, NIST and the Alexander von Humboldt Foundation.

Source: http://www.nist.gov/pml/div689/droplet-022614.cfm

Laser light at useful wavelengths from semiconductor nanowires

Thread-like semiconductor structures called nanowires, so thin that they are effectively one-dimensional, show potential as lasers for applications in computing, communications, and sensing. 

Scientists at the Technische Universität München (TUM) have demonstrated laser action in semiconductor nanowires that emit light at technologically useful wavelengths and operate at room temperature. They now have documented this breakthrough in the journal Nature Communications and, in Nano Letters, have disclosed further results showing enhanced optical and electronic performance.

"Nanowire lasers could represent the next step in the development of smaller, faster, more energy-efficient sources of light," says Prof. Jonathan Finley, director of TUM's Walter Schottky Institute. Potential applications include on-chip optical interconnects or even optical transistors to speed up computers, integrated optoelectronics for fiber-optic communications, and laser arrays with steerable beams. "But nanowires are also a bit special," Finley adds, "in that they are very sensitive to their surroundings, have a large surface-to-volume ratio, and are small enough, for example, to poke into a biological cell." Thus nanowire lasers could also prove useful in environmental and biological sensing.

These experimental nanowire lasers emit light in the near-infrared, approaching the "sweet spot" for fiber-optic communications. They can be grown directly on silicon, presenting opportunities for integrated photonics and optoelectronics. And they operate at room temperature, a prerequisite for real-world applications.

Tailored in the lab, with an eye toward industry

Tiny as they are – a hundred to a thousand times thinner than a human hair – the nanowire lasers demonstrated at TUM have a complex "core-shell" cross-section with a profile of differing semiconductor materials tailored virtually atom by atom.

The nanowires' tailored core-shell structure enables them to act both as lasers, generating coherent pulses of light, and as waveguides, similar to optical fibers. Like conventional communication lasers, these nanowires are made of so-called III-V semiconductors, materials with the right "bandgap" to emit light in the near-infrared. A unique advantage, Finley explains, is that the nanowire geometry is "more forgiving than bulk crystals or films, allowing you to combine materials that you normally can't combine." Because the nanowires arise from a base only tens to hundreds of nanometers in diameter, they can be grown directly on silicon chips in a way that alleviates restrictions due to crystal lattice mismatch – thus yielding high-quality material with the potential for high performance.

Put these characteristics together, and it becomes possible to imagine a path from applied research to a variety of future applications. A number of significant challenges remain, however. For example, laser emission from the TUM nanowires was stimulated by light – as were the nanowire lasers reported almost simultaneously by a team at the Australian National University – yet practical applications are likely to require electrically injected devices.

Nanowire lasers: a technological frontier with bright prospects

The newly published results are largely due to a team of scientists who are beginning their careers, under the guidance of Dr. Gregor Koblmüller and other senior researchers, at the frontier of a new field. Doctoral candidates including Benedikt Mayer, Daniel Rudolph, Stefanie Morkötter and Julian Treu combined their efforts, working together on photonic design, material growth, and characterization using electron microscopy with atomic resolution.

Ongoing research is directed toward better understanding the physical phenomena at work in such devices as well as toward creating electrically injected nanowire lasers, optimizing their performance, and integrating them with platforms for silicon photonics.

"At present very few labs in the world have the capability to grow nanowire materials and devices with the precision required," says co-author Prof. Gerhard Abstreiter, founder of the Walter Schottky Institute and director of the TUM Institute for Advanced Study. "And yet," he explains, "our processes and designs are compatible with industrial production methods for computing and communications. Experience shows that today's hero experiment can become tomorrow's commercial technology, and often does."

This research was supported in part by the German Excellence Initiative through the TUM Institute for Advanced Study and the Excellence Cluster Nanosystems Initiative Munich (NIM); by the German Research Foundation (DFG) through Collaborative Research Center SFB 631; by the European Union through a Marie Curie European Reintegration Grant, the QUROPE project SOLID, and the EU-MC network INDEX; by a CINECA award under the ISCRA initiative; and by a grant from Generalitat Valenciana.

Source: http://www.tum.de/en/about-tum/news/press-releases/short/article/31226/

Stanford Scientists Use DNA to Assemble a Transistor From Graphene

Graphene, a sheet of carbon atoms arrayed in a honeycomb pattern, could be a better semiconductor than silicon.

DNA is the blueprint for life. Could it also become the template for making a new generation of computer chips based not on silicon, but on an experimental material known as graphene?

That’s the theory behind a process that Stanford chemical engineering professor Zhenan Bao reveals in Nature Communications.

Bao and her co-authors, former post-doctoral fellows Anatoliy Sokolov and Fung Ling Yap, hope to solve a problem clouding the future of electronics: consumers expect silicon chips to continue getting smaller, faster and cheaper, but engineers fear that this virtuous cycle could grind to a halt.

Why has to do with how silicon chips work.

Everything starts with the notion of the semiconductor, a type of material that can be induced to either conduct or stop the flow of electricity. Silicon has long been the most popular semiconductor material used to make chips.

The basic working unit on a chip is the transistor. Transistors are tiny gates that switch electricity on or off, creating the zeroes and ones that run software.

To build more powerful chips, designers have done two things at the same time: they’ve shrunk transistors in size and also swung those gates open and shut faster and faster.

The net result of these actions has been to concentrate more electricity in a diminishing space. So far that has produced small, faster, cheaper chips. But at a certain point, heat and other forms of interference could disrupt the inner workings of silicon chips.

"We need a material that will let us build smaller transistors that operate faster using less power," Bao said.

Graphene has the physical and electrical properties to become a next-generation semiconductor material – if researchers can figure out how to mass-produce it.

To the right is a honeycomb of graphene atoms. To the left is a double strand of DNA. The white spheres represent copper ions integral to the chemical assembly process. The fire represents the heat that is an essential ingredient in the technique. (Anatoliy Sokolov) 

Graphene is a single layer of carbon atoms arranged in a honeycomb pattern. Visually it resembles chicken wire. Electrically this lattice of carbon atoms is an extremely efficient conductor.

Bao and other researchers believe that ribbons of graphene, laid side-by-side, could create semiconductor circuits. Given the material’s tiny dimensions and favorable electrical properties, graphene nano ribbons could create very fast chips that run on very low power, she said.

"However, as one might imagine, making something that is only one atom thick and 20 to 50 atoms wide is a significant challenge," said co-author Sokolov.

To handle this challenge, the Stanford team came up with the idea of using DNA as an assembly mechanism.

Physically, DNA strands are long and thin, and exist in roughly the same dimensions as the graphene ribbons that researchers wanted to assemble.

Chemically, DNA molecules contain carbon atoms, the material that forms graphene.

The real trick is how Bao and her team put DNA’s physical and chemical properties to work.

The researchers started with a tiny platter of silicon to provide a support (substrate) for their experimental transistor. They dipped the silicon platter into a solution of DNA derived from bacteria and used a known technique to comb the DNA strands into relatively straight lines.

Next, the DNA on the platter was exposed to a copper salt solution. The chemical properties of the solution allowed the copper ions to be absorbed into the DNA.

Next the platter was heated and bathed in methane gas, which contains carbon atoms. Once again chemical forces came into play to aid in the assembly process. The heat sparked a chemical reaction that freed some of the carbon atoms in the DNA and methane. These free carbon atoms quickly joined together to form stable honeycombs of graphene.

"The loose carbon atoms stayed close to where they broke free from the DNA strands, and so they formed ribbons that followed the structure of the DNA," Yap said.

So part one of the invention involved using DNA to assemble ribbons of carbon. But the researchers also wanted to show that these carbon ribbons could perform electronic tasks. So they made transistors on the ribbons.

"We demonstrated for the first time that you can use DNA to grow narrow ribbons and then make working transistors," Sokolov said.

The paper drew praise from UC Berkeley associate professor Ali Javey, an expert in the use of advanced materials and next-generation electronics.

"This technique is very unique and takes advantage of the use of DNA as an effective template for controlled growth of electronic materials,” Javey said. “In this regard the project addresses an important research need for the field."

Bao said the assembly process needs a lot of refinement. For instance, not all of the carbon atoms formed honeycombed ribbons a single atom thick. In some places they bunched up in irregular patterns, leading the researchers to label the material graphitic instead of graphene.

Even so, the process, about two years in the making, points toward a strategy for turning this carbon-based material from a curiosity into a serious contender to succeed silicon.

"Our DNA-based fabrication method is highly scalable, offers high resolution and low manufacturing cost," said co-author Yap. "All these advantages make the method very attractive for industrial adoption."

The experiment was supported in part by the National Science Foundation and the Stanford Global Climate and Energy Program.

Tom Abate is associate director of communications in the Stanford School of Engineering.

Source: http://engineering.stanford.edu/news/stanford-scientists-use-dna-assemble-transistor-graphene

Laser Spectroscopy Helping to Measure Progress in Nanotech Design

“The interface between two semiconductor materials enables most of the electronic gadgets we use each day, from computers to mobile phones, displays and solar cells,” said Guannan Chen, a graduate student in Drexel’s Materials Science and Engineering department and the lead author of the group’s report, which was recently published in Nano Letters. “One of the most important features of the interface is the height of the energy step required for the electron to climb over, known as band offset. Current methods for measuring this step height in planar devices are not practical for nanoscale devices, however, so we set off to find a better way to make this measurement. 

”Engineers working in the nanoscale will have a new tool at their disposal thanks to an international group of researchers led by Drexel University’s College of Engineering. This innovative procedure could alleviate the persistent challenge of measuring key features of electron behavior while designing the ever-shrinking components that allow cell phones, laptops and tablets to get increasingly thinner and more energy efficient.  

Measuring the band offset faced by electrons jumping from one material to another is a key component of the design process because it guides the redesign and prototyping of nanoscale components in order to make them as efficient and effective as possible.

Using laser-induced current in a nanowire device and its dependence on the wavelength of the laser, the team devised a new method to derive the band offset. As they continuously change the wavelength of the laser, they measure the photocurrent responses. From this data they are able to determine the band offset.

“Using the interface within a co-axial core-shell semiconductor nanowire as a model system, we made direct measurements of the band offset for the first time in nanowire electronics,” Chen said. “This is a significant cornerstone to freely design new nanowire devices such as solar cells, LEDs, and high speed electronics for wireless communications. This work can also extend to broader material systems which can be tailored for specific application.”

The study, which was funded primarily by the National Science Foundation, also included researchers from Lehigh University, National Research Council – Institute for Microelectronics and Microsystems (IMM-CNR) and the University of Salento in Italy, Weizmann Institute of Science and Negev Nuclear Research Center in Israel and the University of Alabama. Each group added a key component to the project.

“Teamwork and close collaborations are essential in this work,” said Guan Sun, the lead researcher from Lehigh. “The smooth channel of sharing ideas and experiment resources is valuable within the team because the quality and variety of the material system is vital to achieving accurate results.”

While Drexel’s members designed the experiments, processed the materials, made the nanowire device and conducted spectroscopic experiments, Sun and Yujie Ding, from Lehigh, supported the research with complementary optical experiments.

The collaborators from the IMM-CNR, Paola Prete, and the University of Salento, Ilio Miccoli and Nico Lovergine joined forces with Hadas Shtrikman, from Weizmann Institute of Science to produce the high quality nanowire used in the testing. Patrick Kung, from the University of Alabama, analyzed the composition of the nanowire at the atomic level, and Tsachi Livneh, of Negev Nuclear Research Center, contributed to the analyses.

“This remarkably simple approach to obtaining a key characteristic in individual nanowires is an exciting advance,” said Dr. Jonathan Spanier, a professor in Drexel’s College of Engineering who is the lead investigator of the project. “We anticipate it will be a valuable method as we develop nanoscale electronic devices having completely new and important functionalities.”

With a better understanding of the material and electron behavior, the team will continue to pursue novel nanoscale optoelectronic devices such as new-concept transistors, electron-transfer devices and photovoltaic devices.

NEWS MEDIA CONTACT

BRITT FAULSTICK 

News Officer, University Communications

britt.faulstick@drexel.edu
Phone: 215-895-2617
Mobile: 215-796-5161

http://www.drexel.edu/now/news-media/releases/archive/2013/September/measuring-band-gaps/#sthash.jZ5iQcFB.dpuf

Cost-saving computer chips get smaller than ever

Not so long ago, a computer filled a whole room and radio receivers were as big as washing machines. In recent decades, electronic devices have shrunk considerably in size and this trend is expected to continue, leading to enormous cost and energy savings, as well as increasing speed. 

Key to shrinking devices is Terascale computing, involving ultrafast technology supported by single microchips that can perform trillions of operations per second. 

Using Terascale technology, semiconductor components commonly used to make integrated circuits for all kinds of appliances could measure less than 10 nanometers within several years. Keeping in mind that a nanometer is less than 1 billionth of a meter, electronic devices have the potential to become phenomenally smaller and require significantly less energy than today - a development that will revolutionise the electronics industry. 

Despite progress, the technology for producing these ultra-small devices has a long way to go before being reliable. To advance the work, the EU-funded project TRAMS ('Terascale reliable adaptive memory systems') sought to improve reliability by improving chip design. 

The TRAMS team conducted in-depth variability and reliability analyses to develop chip circuits that are much less prone to errors. These circuits feature new designs that yield reliable memory systems from currently unreliable nanodevices. 

The main challenge was to develop reliable, energy efficient and cost effective computing using a variety of new technologies with individual transistors potentially measuring below five nanometers in size. 

The team investigated a number of technologies and materials with potential to make Terascale computing a reality. These included: 

- carbon nanotubes (very tiny cylindrical nanostructures grapheme technology); 
- new transistor geometries, such as FinFETs; 
- state-of-the-art nanowires, which offer very advanced transistor capabilities for use in a new generation of electronic devices. 

Using models, the researchers analysed reliability - from the technology to the circuit level. 

These advances are expected to redefine today's standard 'complementary metal-oxide semiconductors' (CMOS). The team's results would help Europe's manufacturers develop CMOS devices below the 16 nanometre range. The biggest challenge will lie in reducing CMOS devices to below five nanometres - a development that now starts to look possible. 

From communication and security to transport and industry, CMOS-based devices of the future promise to redesign the technology we use, introducing radical energy and cost savings. 

The TRAMS consortium includes universities and companies from Spain, Belgium and the UK. The project was coordinated by Spain's Universitat Politècnica de Catalunya, and received almost EUR 2.5 million in EU funding. The team concluded its work in December 2012.

http://trams-project.upc.edu/